Various portable electronic equipments (cell phones, portable information terminals PDA, note-type personal computers, portable audio/video players, digital cameras, digital video cameras, etc.) usually use batteries as power supplies, comprising DC-DC converters for converting power supply voltage to operation voltage. The DC-DC converter is generally constituted by integrated semiconductor circuits (active parts) including switching devices and control circuits, inductors (passive parts), etc. disposed as discrete parts on a printed circuit board.
For the miniaturization of electronic equipments, the DC-DC converter has an increasingly higher switching frequency, using more than 1 MHz at present. Because semiconductor devices such as CPU are getting higher in speed, function and current and lower in operating voltage, low-voltage, high-current DC-DC converters are needed.
Passive parts used in power supply circuits for DC-DC converters, etc. are required to be smaller in size and height, and integrated with active parts. The inductor, one of passive parts, has conventionally been composed of a wire wound around a magnetic core, and its miniaturization is limited. Because lower inductance is needed in order that laminate devices are operable at higher frequencies, monolithic laminate devices having a closed magnetic path structure have become used.
The laminated inductor, an example of laminate devices, is produced by integrally laminating magnetic material (ferrite) sheets printed with coil patterns, and sintering them. The laminated inductor has excellent reliability with little magnetic flux leakage. However, because it has an integral structure, magnetic saturation partially occurs in a magnetic material in the laminated inductor by a DC magnetic field generated when a magnetization current is applied to the coil pattern, resulting in drastic decrease in inductance. Such laminated inductors have poor DC-superimposed characteristics.
To solve this problem, JP 56-155516 A and JP 2004-311944 A disclose a laminated inductor 50 having an open magnetic path structure comprising a magnetic gap layer between magnetic layers, as shown in FIG. 47. This laminated inductor 50 is formed by laminating pluralities of magnetic (ferrite) layers 41 with coil pattern layers 43, the magnetic gap layer 44 made of a non-magnetic material being inserted into a magnetic path. In the figure, a magnetic flux is schematically shown by arrows. At small magnetization current, a magnetic flux φa flowing around each coil pattern 43, and a magnetic flux φb flowing around pluralities of coils patterns 43 are formed in each of regions separated by the magnetic gap layer 44. Most magnetic fluxes do not pass through the magnetic gap layer 44, but a magnetic flux path is formed in each region separated by the magnetic gap layer 44, as if two inductors were series-connected in one device. At large magnetization current, on the other hand, material portions between the coil patterns 43 are magnetically saturated, so that most magnetic fluxes pass through the magnetic gap layer 44 like the magnetic flux φc, and flow around pluralities of coils patterns, resulting in a demagnetizing field that lowers inductance than in the case of small magnetization current. However, the laminated inductor becomes resistant to magnetic saturation. Thus, the conventional laminated inductor has DC-superimposed characteristics improved by the magnetic gap layer, but its inductance largely varies by slight increase in magnetization current. Although the DC-superimposed characteristics are improved as compared with when the magnetic gap layer 44 is not formed, further improvement is needed so that the laminated inductor is operable at large magnetization current.
JP 2004-311944 A discloses a laminated inductor 50 comprising a magnetic gap layer 44 embedded at center between coil patterns, and a non-magnetic body 47 embedded around the coil patterns, as shown in FIG. 48. Because most magnetic fluxes pass through the magnetic gap layer 44, this laminated inductor 50 has stable inductance in a range from small magnetization current to large magnetization current, but exhibits insufficient performance at large magnetization current. In addition, it is difficult to produce because of a complicated structure.